Functions as First-Class Objects in JavaScript: Why Does This Matter?

Functions in JavaScript are first-class objects (or “first-class citizens”). Fascinating, but… what does that mean? Why does it matter? Read on and we’ll have a look!

We’ll start with the basics: What does first-class citizenship mean in general? First-class citizenship, within the world of programming, means that a given entity (such as a function) supports all the operational properties inherent to other entities; properties such as being able to be assigned to a variable, passed around as a function argument, returned from a function, etc. Basically, first-class citizenship simply means “being able to do what everyone else can do.”

In JavaScript, functions are objects (hence the designation of first-class object). They inherit from the Object prototype and they can be assigned key: value pairs. These pairs are referred to as properties and can themselves be functions (i.e., methods). And as mentioned, function objects can be assigned to variables, they can be passed around as arguments; they can even be assigned as the return values of other functions. Demonstrably, functions in JavaScript are first-class objects.

Let’s look at some examples/tests:

Can we assign a function to a variable?

See the Pen JRVgLw by Develop Intelligence (@DevelopIntelligenceBoulder) on CodePen.

Yes we can!

Can we pass a function as an argument to another function?

See the Pen XjQvqY by Develop Intelligence (@DevelopIntelligenceBoulder) on CodePen.

Sure enough!

But can we return a function… from a function?? (Hint: We already did, but… let’s see it again!)

See the Pen LRvwro by Develop Intelligence (@DevelopIntelligenceBoulder) on CodePen.

Yep, piece of cake!

One can get pretty creative with assigning functions to variables and passing them around to other functions from which they can be returned. If you’re not careful (or maybe if you just want to have a bit of fun!), the rabbit hole can get pretty deep, pretty quickly! Consider this… a function can be passed to itself and even returned from itself!

Excellent! But okay… who cares if JavaScript functions are first-class objects? What does it matter?

The beauty of JavaScript functions enjoying first-class citizenship is the flexibility it allows. Functions as first-class objects opens the doors to all kinds of programmatic paradigms and techniques that wouldn’t otherwise be possible. Functional programming is one of the paradigms that first-class functions allow. Additionally, listening for and handling multiple events by passing callback functions is a useful feature within JavaScript and is achieved by passing a function as an argument to the document object’s addEventListener method. The process would not be nearly as elegant if functions were not granted first-class citizenship within the language. Furthermore, the practices of closure and partial-application/currying would not be possible within JavaScript if functions didn’t enjoy the status of first-class.

In summation, with functions being first-class objects within JavaScript, developers are able to do all kinds of interesting things and explore all sorts of programming paradigms that wouldn’t be otherwise be possible. It is in part due to this functional first-classness that JavaScript has become the powerful and prolific language that it is today.

Thanks for reading!